This invention relates to a method of cold pressing a plurality of superposed sheets. Where an element harmfully affected by heat such as a semiconductor element, is sealed, the conventional process comprises sealing a vessel of said semiconductor element by cold pressing.
An important factor of governing the adhesiveness of an object subjected to cold pressing is the plastic fluidity of that part of said object which undergoes pressure. Where superposed discs 1a, 1b of FIG. 1 are pressed together at an annular junction 2, then two opposing stresses arise inside of said annular junction 2 due to deformation resulting from the plastic fluidity of said discs 1a, 1b at said annular junction 2. These stresses act to obstruct the plastic fluidity of said pressed discs 1a, 1b at said annular junction 2, thereby raising the problems that the adhesivity of the superposed discs 1a, 1b at said annular junction 2 drops to bring about unsatisfactory adhesion. Where the superposed discs 1a, 1b have a great thickness, or the annular junction 2 is widely spaced from the outer edges of the superposed discs 1a, 1b, then deformation resulting from the plastic fluidity of said discs 1a, 1b at said annular junction 2 gives rise to a force which tends to expand the outer portion of the annular junction 2, that is, the occurrence of a reactionary stress. This reactionary stress caused by said deformation act to obstruct the plastic fluidity of the superposed discs 1a, 1b at the annular junction 2. Consequently the same problem is raised on the outside of the annular junction 2, as on the inside thereof.
Therefore, the conventional process of minimizing the decrease of pressure adhesivity caused by deformation resulting from the plastic fluidity of a plurality of superposed sheets, for example, at the annular junction, comprises elevating the workability of superposed sheets, as much as possible. (Said workability is herein defined to mean the extent to which the thickness of superposed sheets at the annular junction can be reduced.) Where, however, superposed sheets fabricated with high workability are pressed together, then the joint of said superposed sheets has a reduced thickness, resulting in a decline in the mechanical strength of a product obtained by pressure adhesion.